In view of the fact that earthquakes occur suddenly and generally without much warning, it is necessary for recorders designed to record the occurrence of sesimic events to be continuously operational and fully prepared to record even the beginnings of the seismic signatures. For this reason mechanical recorders have proven impractical. Not only do such machines tend to fail to function properly after many months of non-operation, but in every case their response time to the sudden discontinuity between a long period of dormancy and the requirement of immediate precise recording of an event is so slow that the elapsed time required to bring up to speed a recording medium, such as a photographic film or magnetic tape, usually results in the loss of the beginning of the seismic signature of interest. Conversely, the storage of digitized data in active electronic memory chips involves no moving parts and therefore results in no start-up loss of the initial portions of an event of interest. This type of storage can be done in such a way that all seismic events are continuously recorded, with events of no interest being written-over by newly occurring events until an event of real interest occurs, at which time the record of such an event is transferred to an electronic memory where it is retained rather than being written-over by subsequent events. There are no moving parts except within the three orthogonal seismometers themselves.
U.S. Pat. No. 4,409,670 to Herndon shows a digital flight-data recorder which receives and temporarily stores multiple diverse digitized flight parameters, the system compressing and more permanently storing those data frames which contain flight parameters which are of significant interest, the data being compressed and formatted to achieve a smaller recorded data base. This patent uses a compression scheme differing fundamentally from that of the present invention.
U.S. Pat. No. 3,990,036 to Savit shows seismic data being collected and held in digital storage registers at each of a number of remote sensor stations and then collected by polling the stations from a central processor.
U.S. Pat. No. 3,790,925 to Ahrens shows a digital store used in an echo sounder and subsequently read out when required.
In U.S. Pat. No. 4,323,990 to Goode et al, seismometer signals are digitized by an A/D converter and specially formatted for recording, but the recording is on magnetic tape, even for short-term storage, and thus would suffer from most of the mechanical disadvantages related above when used in a system that continously monitors for earthquakes, month after month.
The U.S. Pat. No. 4,300,135, to Korn et al, shows an earthquake monitoring system using a microprocessor, but this circuit triggers an alarm rather than recording the signature of the quake.